Method and device for sludge treatment
The present invention relates to an automatic and continuous method of converting heavy metal hydroxides, which are an environmental hazard and are poisonous, and which are obtained in sludge from purification works for pickling, rinsing and surface-treatment plants, into heavy metal oxides in powder and/or granules.
There are many industrial processes for which fresh water is required, where the water, after having been used, contains various pollutants that are carried along with the wastewater into rivers and streams, unless they pass through municipal purification works. In special cases of pollution, it is inappropriate to allow the wastewater to be fed into municipal purification works. This applies particularly when the industrial waste contains heavy metals such as lead, copper, chromium, nickel, cadmium, zinc etc.
The metals most often occur together with acids, and then form very poisonous compounds. These compounds occur in pickling and surface-treatment processes, which require for the subsequent rinsing processes much fresh water and result in a highly polluted wastewater.
If suitable acid and metal recovery devices are not available, the used bath water is discharged into the sewer system together with the washing water.
Industries are now required, to an increasing degree, to arrange for wastewater treatment before the waste water is discharged into the municipal purification works or out into rivers and streams.
This treatment usually consists of neutralizing the acids with the aid of basic additives and with the aid of special additives (flocculation agents) to precipitate the dissolved metals in hydroxide flocks, which are either allowed to sediment in decantering vessels or are skimmed off in flotation plants, whereupon the treated water is harmless for discharge into the municipal purification works or into some river or stream.
The hydroxide sludge, which contains 96 - 98 % water, is still poisonous and constitutes an environment hazard, and as simple methods of destroying the sludge are not available in the market at present, the companies have great difficulty in disposing of it. Central collection places have been arranged, to which the sludge is conveyed with tank trucks. There are, of course, risks involved in transporting considerable quantities of poison on public roads. Moreover, it is unsatisfactory that large quantities of poison are concentrated in certain places, as the risk for unintentional discharging through leakage increases as the stocks grow continuously since there is no suitable destruction equipment available.
From an environmental point of view it would be most appropriate to neutralize dangerous poisons and waste at the source, so that the residual products will be entirely harmless and, if possible, can either be subjected to recovery processes or else can be deposited among ordinary municipal waste.